refactor(build): restructure into separate server targets

Update xmake.lua to split into HTTP, file, and DNS server targets with
individual source directories. Remove old main.cpp and add test.txt for new
functionality.

src/main.cpp

@@ -1,16 +0,0 @@
-#include <boost/asio.hpp>
-#include <iostream>
-
-int main()
-{
-    std::cout << "Hello, Boost + C++23 + xmake!\n";
-
-    boost::asio::io_context io;
-    boost::asio::steady_timer timer(io, std::chrono::seconds(1));
-    timer.async_wait([](const boost::system::error_code &) {
-        std::cout << "Timer fired after 1 second!\n";
-    });
-
-    io.run();
-    return 0;
-}

www/test.txt

@@ -0,0 +1 @@
+Hello from file!

xmake.lua

@@ -2,16 +2,37 @@ set_languages "c++23"
 
 add_rules("plugin.compile_commands.autoupdate", { outputdir = "." })
 
-add_requires("boost", {
-    system = true, -- sudo pacman -S boost
-
+add_requires("cmake::Boost", {
+    alias = "boost",
+    system = true,
     configs = {
-        all = false,
-        system = true, -- asio
+        components = { "program_options", "log" },
     },
 })
 
-target "app"
-set_kind "binary"
-add_files "src/*.cpp"
-add_links "boost_system"
+-- HTTP server target
+target "http_server"
+do
+    set_kind "binary"
+    add_files("src/main/main_http.cpp", "src/http/*.cpp", "src/common/*.cpp")
+    add_includedirs("src", "src/http", "src/common", { public = true })
+    add_packages "boost"
+end
+
+-- File server target
+target "file_server"
+do
+    set_kind "binary"
+    add_files("src/main/main_file.cpp", "src/file/*.cpp", "src/common/*.cpp")
+    add_includedirs("src", "src/file", "src/common", { public = true })
+    add_packages "boost"
+end
+
+-- DNS server target
+target "dns_server"
+do
+    set_kind "binary"
+    add_files("src/main/main_dns.cpp", "src/dns/*.cpp", "src/common/*.cpp")
+    add_includedirs("src", "src/dns", "src/common", { public = true })
+    add_packages "boost"
+end